Communication to residences and commercial structures typically use a twisted pair of copper wires for telephone voice band transmission in a frequency range of 300 to 3,400 Hz. When a personal computer or similar digital communication device having a modem is in use, the telephone line cannot be used for a normal telephone conversation because the modem uses the bandwidth assigned to the voice band transmission.
However, a twisted pair telephone line may have a bandwidth of several MHZ. Unfortunately in the past, the voice band frequency was the only bandwidth used with these lines because the equipment at the telephone company would only pass the voice band. Now, modern telephone equipment can support higher transmission rates. Still, because of the characteristics of the entire channel, end user telephone and data communication equipment typically use the same voice band.
At higher frequencies, especially in the MHZ frequency range, the signals are greatly attenuated especially when transmitted over distances up to several thousand meters. To overcome this, it is necessary that any transmitter, such as a modem and its associated circuitry, transmits the signal at a much higher power than is usual to account for the signal attenuation. By using a frequency range above a voice frequency band for some transmission, such as data transmission, it is possible to simultaneously use the telephone for normal voice communication.
For example, for a typical Internet user, only a few command signals are generated and transmitted to the Internet Service Provider on the uplink. At the same time, however, signals having a wide bandwidth may be received. In other words, the lower bandwidth requirement on the uplink and higher bandwidth requirement on the downlink defines an asymmetric communication system. Because the frequencies above voice band are subject to greater attenuation, transmitter power may be increased during or prior to transmission. Unfortunately, the increased transmitter power does create some difficulties, such as local echo which may interfere with the received signals. This unwanted echo may be generated by various means, such as any impedance mismatch between the transmitter and the twisted pair telephone line or other equipment connected to the line. These problems occur in duplex transmission when one wants to transmit and receive at the same time.
One prior art approach to address the unwanted echo signal has provided relatively complicated analog filters in the circuitry of the modem. Typically, these filters may include a voice band and high frequency filters, as well as various receiver and transmitter filters. These systems require high order analog filtering to remove the effects of the transmitted signal from the receive band.
Also, a receiver's analog-to-digital converter may need a very large dynamic range so that the composite received signal (the received signal including the echo) does not saturate the analog-to-digital converter. It is also important to sample the desired signal with enough resolution to meet the required signal-to-noise ratio. This requirement is difficult to achieve in practice.
Another prior art approach has been to generate a scaled replica of the transmitted signal, and subtract the replica from the received signal to thereby remove or reduce the unwanted echo. In such a prior art device, a voltage scaler is operatively connected to the transmitter to generate the scaled replica of the transmit signal which, in turn, is fed to an inverting input of an amplifier to cancel the unwanted echo based on the transmitted signal. Typically, this type of device and method are limited because it was assumed that half of the power was transmitted to the telephone line, while half the power was reflected back into the receiver as unwanted interference in the entire frequency band. This system also anticipated a fixed line impedance (e.g., 110 Ohms) to obtain cancellation of the unwanted echo signal before it reached the analog-to-digital converter of the receiver.
As the line impedances can typically vary by .+-.20%, thus, the local echo may have a different amplitude than that produced by the scaler configured for a fixed line impedance. Additionally, the replicated echo cancellation signal may be ineffective due to amplitude and phase differences between the unwanted transmitter signal forming the echo and the replicated signal. This is because the scaler only tries to match the resistive component and ignores the inductive and capacitive components of the line impedance. To overcome this in the past has meant designing complex analog filters and high-dynamic range A/D's as mentioned previously.